1. Field of the Invention
This invention relates to a method of correcting a read-out error of image signals read out by scanning a stimulable phosphor sheet carrying a radiation image stored therein, and more particularly to a method of correcting an image signal read-out error caused, when a stimulable phosphor sheet carrying a radiation transmission image stored therein is scanned by stimulating rays by use of a multifaceted deflector such as a multifaceted mirror or a hologram scanner and light emitted by the stimulable phosphor sheet in proportion to the stored radiation energy upon exposure to the stimulating rays is photoelectrically read out and converted into image signals, by distortion and/or shift in scan lines due to unevenness in the surfaces of the multifaceted deflector and/or incorrect orientation of the surfaces of the same.
2. Description of the Prior Art
When certain kinds of phosphors are exposed to a radiation such as X-rays, .alpha.-rays, .beta.-rays, .gamma.-rays or ultraviolet rays, they store a part of the energy of the radiation. Then when the phosphor which has been exposed to the radiation is exposed to stimulating rays such as visible light, light is emitted by the phosphor in proportion to the stored energy of the radiation. A phosphor exhibiting such properties is referred to as "a stimulable phosphor".
It has been proposed, for instance, in U.S. Pat. No. 4,258,264 and Japanese Unexamined Patent Publication No. 56(1981)-11395, to use a stimulable phosphor in a radiation image recording and reproducing system. Specifically, a sheet provided with a layer of the stimulable phosphor (hereinafter referred to as "a stimulable phosphor sheet" or simply as "a sheet") is first exposed to a radiation passing through an object such as a human body to have a radiation image stored therein, and is then scanned with stimulating rays such as a laser beam which cause it to emit light in the pattern of the stored image. The light emitted by the stimulable phosphor sheet upon stimulation thereof is photoelectrically detected and converted into an electric image signal, which is processed as desired to reproduce a visible image on a recording medium such as a photographic light-sensitive material or on a display device such as a cathode ray tube (CRT).
Generally the stimulating laser beam is deflected to scan the stimulable phosphor sheet by a multifaceted deflector or a galvanometer mirror. In case that a multifaceted deflector is employed, there arises a problem that if the orientation of the deflecting surfaces is not correct or if there is unevenness in the deflecting surfaces, the positional relationship between adjacent scan lines becomes incorrect and an image signal read-out error is caused. That is, if the orientation of the deflecting surfaces of the multifaceted deflector is not proper, the center to center distances between adjacent scan lines fluctuate, which leads to an image signal read-out error as will be described in more detail hereinbelow. Further, unevenness in the surfaces of the multifaceted deflector causes distortion of the scan lines, which also leads to an image signal read-out error.
Since the width of each scan line is very small, e.g., about 100 .mu.m, the orientation of each surface of the multifaceted deflector and the surface evenness thereof must be controlled very precisely. Though the surface evenness of each surface of the multifaceted deflector can be controlled to such an extent to permit obtaining substantially straight scan lines having a width of approximately 100 .mu.m, it is very difficult to manufacture a multifaceted deflector having properly oriented deflecting surfaces in which the angle of inclination of each surface from its predetermined position with respect to the rotational axis thereof (This angle will be simply referred to as "the inclining angle", hereinbelow.) is smaller than about 5 seconds. When the inclining angle of each deflecting surface of the multifaceted deflector is 5" or more, the scan line drawn on the stimulable phosphor sheet by the deflecting surface is shifted by some dozen .mu.m from the position in which it would be drawn if the inclining angle of the deflecting surface were zero. The shift of some dozen .mu.m of the scan line substantially influences the value of the obtained image signal as can be readily understood from the fact that the width of each scan line is as small as 100 .mu.m.
Conventionally, an optical means such as a light deflector or a cylindrical lens has been used for adjusting the incident angle or the reflecting angle of the stimulating laser beam with respect to the multifaceted deflector, thereby compensating for the incorrect orientation of the deflecting surfaces of the multifaceted deflector. However, when the light deflector is used for such a purpose, the light deflector must be controlled in synchronization with each deflecting surface of the multifaceted deflector, and when other optical means is used, the structure of the system becomes complicated. Therefore, it has been difficult to minimize the size of the system and to reduce the manufacturing cost thereof.